During fetal life, the renin-angiotensin system (RAS) has multiple vital roles including influencing kidney growth, renal vascular development and regulating fetal arterial blood pressure under basal conditions and in times of intrauterine stress. Currently, our understanding of the ontogeny of this system is incomplete. In particular, there are major gaps in our understanding of the maturation of local mechanisms which modulate renin secretion. One local modulator which may be quite important in this regard is nitric oxide (NO) produced by type l nitric oxide synthase in the macula densa. In this application, we will test three inter-related hypotheses concerning the role of NO. Briefly, these hypothesis are: i) that NO is a positive modulator of renin secretion in the mature fetus with a minimal role in the immature animal; 2) that NO exerts its effects by two pathways, a direct one involving inhibition of phosphodiesterase III in juxtaglomerular cells and an indirect one involving endoperoxide synthetase in the macula densa; and 3) that chronic manipulations of the macula densa in the immature fetus will promote the stimulatory effect of NO. We will use fetal sheep to test these hypotheses because we can study this model in utero in the absence of stress and anesthesia. Also the kidneys of these animals are large enough to provide sufficient amounts of renal cortical tissue for our in vitro studies without requiring excessive numbers of animals. We will use western blotting, immunocytochemistry, assays of enzyme activity and RNase protection assays to study the expression of NOS-l, COX-2, and renin in fetal kidneys at two stages of development and following chronic manipulations in vivo. We will study plasma renin responses to pharmacological and physiological interventions in order to establish a role for NO in vivo. We will use pharmacological approaches in vitro to define the mechanisms whereby NO can stimulate the RAS. To our knowledge, these will be the first studies to systematically examine the maturation of the ability of NO to modulate renin secretion and mechanisms by which this modulation is accomplished. Thus, the studies will provide important new information on the role of NO in the modulation of renin secretion during development and, by extension on the maturation of the macula densa as a regulator of renin expression. Understanding more about the development of these inter- relationships is important because derangement of the timely establishment of the stimulatory influence of NO could have adverse effects on kidney development. Impaired fetal kidney growth and maturation of renal function can result later in hypertension in both the infant and adult.